It has for many years been conventional to use recirculating toilet systems in aircraft because a relatively large amount of liquid is required to transport the waste material in a gravity toilet system, and by recirculating the liquid the total quantity of liquid required to operate the system is reduced. Recirculating toilet systems are subject to the disadvantage that the recirculating liquids that they employ are corrosive, and therefore leakage from a recirculating toilet system may enable the corrosive liquid to contact structural members of the aircraft, resulting in the structural integrity of the aircraft being impaired.
Vacuum toilet systems have been known for many years. The modern vacuum toilet system comprises a waste-receiving bowl, a sewer pipe that can be placed under a pressure that is substantially lower than that in the interior of the waste-receiving bowl, and a discharge valve for controlling passage of material from the waste-receiving bowl into the sewer pipe. When the discharge valve is opened, material in the waste-receiving bowl is transported into the sewer pipe by virtue of the pressure difference between the interior of the waste-receiving bowl and the interior of the sewer pipe. Material that enters the sewer pipe by way of the discharge valve is transported through the sewer pipe to a container. The system also comprises a source of pressurized rinse liquid and a rinse liquid valve for controlling introduction of rinse liquid into the waste-receiving bowl.
Vacuum toilet systems do not lend themselves to recirculation, because of the large pressure difference between the downstream side of the discharge valve and the upstream side of the rinse liquid valve. However, because vacuum toilet systems rely on vacuum for removal of the waste material from the bowl, the amount of rinse liquid that is needed in a vacuum toilet system is much smaller than the amount of rinse liquid required in a gravity toilet system. Consequently, non-recirculating vacuum toilet systems employing water as rinse liquid are attractive for use in aircraft.
When a vacuum toilet system is installed in a passenger aircraft, it is conventional for the rinse liquid to be provided from the aircraft's potable water system. The potable water system includes a tank which is positioned below the aircraft's passenger deck, pipes connecting the tank to the consuming devices, such as the galleys and toilets, and a pump for maintaining the water in the pipes under pressure. It is necessary from time to time to drain the potable water system and refill it.
The rinse water valve that is conventionally used in an aircraft vacuum toilet system has a valve closure member which is positioned in a control chamber for cooperating with a seat which surrounds a control port at the bottom of the control chamber. In normal operation of the rinse water valve, a solenoid is energized to bring about movement of the closure member away from the seat, so that the rinse water is allowed to flow through the control port. On de-energizing the solenoid, the pressure drop in the control chamber due to flow past the closure member causes the closure member to be displaced into engagement with the seat. In order to facilitate pressurization of the branch pipe leading to the rinse water valve after the potable water system has been drained and refilled, the closure member is biased away from the valve seat by a light spring. Therefore, as the potable water system is pressurized, air is able to escape from the branch pipe by way of the control port. When the control chamber is filled with water, continued flow past the closure member creates a pressure drop which causes the valve to close against the bias provided by the spring.
It has been found in practice that this type of rinse water valve is not satisfactory, because if the rate of flow of water through the control chamber containing the closure member is small, the pressure drop past the closure member might not be sufficient to overcome the bias of the spring, with the result that the valve does not close. Consequently, water flows through the control port and into the waste-receiving bowl until the malfunction of the valve is detected and remedied. Unless the discharge valve is open, the water that enters the bowl cannot leave the bowl except by overflowing.